1. Field of the Invention
The present invention relates generally to acoustic resonators and, more specifically, the present invention relates to a method and apparatus for a tapered electrode in an acoustic resonator device.
2. Background Information
Acoustic resonators are often used as filters in wireless communication devices. Common types of acoustic resonators include Semiconductor Bulk Acoustic Resonators (SBAR) and Film Bulk Acoustic Resonators (FBAR). An FBAR includes a thin film of piezoelectric (PZ) material positioned between two conductive electrodes. Generally, an air cavity is formed below the bottom electrode. Aluminum Nitride (AlN) and Zinc Oxide (ZnO) are often used as piezoelectric material.
When an electrical signal, such as a Radio Frequency (RF) signal, is applied across the FBAR, the PZ layer expands and contracts, creating a vibration. This vibration creates a mechanical energy (resonance). The fundamental resonance is observed when the thickness of the PZ layer is equivalent to half the wavelength of the input signal.
When multiple FBAR resonators are combined, they can be used to produce a passband filter or a stopband filter. An FBAR can be used as a filter since it will function as an electronic resonator when allowed to operate at its mechanical resonant frequency. FBARs resonate at GHz frequencies and are sized at the micron level, thus making them ideal for wireless communication devices.
A prior art FBAR is shown in FIG. 1. A dielectric layer 12 is formed over substrate 10. Positioned on top of dielectric 12 is a bottom electrode 14. Bottom electrode 14 has a nearly vertical edge making an abrupt end 15. Formed on top of the bottom electrode 14 is piezoelectric layer 16. Positioned on the piezoelectric layer 16 is a top electrode 18. A cavity 20 is formed in substrate 10 and dielectric 12, the top of the cavity 20 defined by bottom electrode 14.
AlN is a well-known ceramic piezoelectric material. When the AlN layer is deposited, it follows the terrain of the under layer and has a tendency to crack when layered over sharp topography. Even small steps of 500 Angstroms in the under surface may cause the AlN to crack. Referring again to FIG. 1, a crack 22 has developed due to the abrupt end 15 of the bottom electrode 14. Cracks in the PZ layer decrease device yield and thus raise the costs of FBAR production.
Also, an under layer with sharp topography affects the crystal orientation of AlN. In FIG. 1, the abrupt edge 15 has caused disorientation in grains 24 of the AlN layer. If the AlN grains are not highly oriented, then the FBAR will experience acoustic losses.